Simply put, I thought of this last night, and then I just noticed it on a website. Blend the URL at the bottom to be the same colour as the AdSense background & page background? I figure its against TOS, but I've never seen it come up on here before.
Don't forget too that some of the ad formats (leaderboard, banner, half banner) do not display the URI, just the title and description.
Even if you wanted to, you can't just change the color. Google will default it to a bright color if the color you chose is the same (or too close) to your page's background. You'll have to use some fancy script to get it to work the way you're talking about, and that is definitely "modifying" the codes and against TOS. By the way, where did you see this being used? I'd like to see how they did it, because it's definitely against TOS if what you're saying is true, that they have successfully blended their URL into the background so that it's "invisible".
Ahhh, that explains it, I saw it on a 460x68 banner. BUT, here is a screen shot, this time without the URL attached to show where it came from since I know the guy, but the 728 leaderboard also isn't showing the URL THIS time... Heres a screen shot.
Leaderboards don't show URLs either. You can view this page to see what does and doesn't include the URLs. https://www.google.com/adsense/adformats